The power assist apparatus is connected between an AC power system and a load and used as an assist for the load power. This power assist apparatus includes a power storage device which combines a step-up/down (buck-boost) chopper and an electricity (or is energy) storage device such as an electric double layer capacitor, and which is connected with a DC circuit of an inverter. The power assist is a term used here to include peak-cut, load leveling, power storage and instantaneous voltage decrease compensation. Such power assist apparatus are disclosed in a patent document 1 and a patent document 2.
FIG. 4 is a schematic view showing the structure of a power assist apparatus. An inverter 1 includes a forward converting section 2 for converting AC to DC, and a bidirectional converting section 3 having bidirectional converting function. The inverter 1 is connected between a commercial power source 4 and a load 5. A step up/down chopper 6 and an electricity storage device 7 form a power storage device, which is connected with the DC circuit of the inverter 1. This power assist apparatus charges the electricity storage device 7 through chopper 6 when, for example, the DC voltage of inverter 1 increases in a regenerative state of load 5 and the DC voltage of inverter 1 becomes higher than or equal to a predetermined charging voltage, and supplies energy stored in electricity storage device 7 to load 5 through chopper 6 and bidirectional converting section 3 when the voltage of commercial power source 4 decreases.    Patent Document 1: JP 2003-244840A    Patent Document 2: JP H11 (1999)-27874A